CHaprer II. 
Stems and Branches. 
THE stem is the distinguishing characteristic of trees, 
separating them from all other groups of plants. Although 
in the region covered by this book the trees include all 
the very large plants, size alone does not make a tree. 
A plant with a single trunk of woody structure that 
does not branch for some distance above the ground, is 
ealled a tree. Woody plants that branch directly above 
the soil, even though they grow to the height of twenty 
feet or more, are called shrubs, or, in popular language, 
bushes. Many plants which have a tendency to grow into 
the form of shrubs may, by pruning, be forced to grow 
tree-like; some that are shrubs in the northern States are 
trees further south. 
All the tees that grow wild, or can be cultivated out of 
doors, in the northern States belong to one class, the stems 
having a separable bark on the outside, a minute stem of 
pith in the center, and, between these, wood in annual 
layers. Such a stem is called exogenous (outside-growing), 
because a new layer forms on the outside of the wood 
each year. 
Another kind of tree-stem is found abundantly in the 
tropics; one, the Palmetto, grows from South Carolina to 
Florida. While in our region there are no trees of this 
character, there are plants having this kind of stem, the 
best illustration being the corn-stalk. In this case there 
is no separable bark, and the woody substance is in threads 
within the pithy material. In the corn-stalk the woody 
threads are not very numerous, and the pith is very abun- 
dant; in most of the tropical trees belonging to this group 
the threads of wood are so numerous as to make the ma- 
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